Monosodium glutamate (MSG) is a widely used flavor enhancer that provides the savory taste known as umami. It is the sodium salt of glutamic acid, an amino acid naturally present in many foods like tomatoes and cheeses. Although MSG has been used in cooking for over a century, its consumption is sometimes linked to various physical symptoms, including headaches, flushing, and gastrointestinal distress. This article investigates the current scientific understanding of the direct link between consuming MSG and experiencing diarrhea.
Scientific Consensus on MSG and Gastrointestinal Distress
Scientific bodies worldwide, including the U.S. Food and Drug Administration (FDA), classify MSG as “generally recognized as safe” (GRAS). This determination is based on extensive research that has repeatedly failed to establish a consistent, clear causal relationship between typical MSG intake and the reported adverse symptoms.
The body metabolizes glutamate from MSG identically to the larger amount found naturally in protein-rich foods. An average adult consumes about 13 grams of naturally occurring glutamate daily, while intake of added MSG is estimated at only about 0.55 grams per day. Glutamate is a major oxidative fuel for the intestinal lining cells, known as enterocytes.
Enterocytes utilize most ingested glutamate for energy and metabolism during its “first pass” through the gut. Because of this extensive metabolism in the intestinal tract, very little glutamate from MSG is absorbed into the bloodstream. This efficient metabolic process is why MSG does not typically trigger systemic digestive reactions like diarrhea in the population at large.
The Role of Individual Sensitivity and Intolerance
While MSG is safe for most, a small subset reports adverse reactions, which are categorized as MSG symptom complex. These reactions are considered a food intolerance rather than a true IgE-mediated food allergy. Gastrointestinal symptoms like bloating, abdominal pain, and diarrhea are among the complaints reported by sensitive individuals.
These reactions are highly dose-dependent, and symptoms are typically mild and transient. They only occur with the rapid consumption of a very large amount of MSG. The threshold for a reaction is generally 3 grams or more consumed without food, which is significantly higher than the amount in a typical single serving.
Controlled, double-blind studies involving individuals who self-identify as MSG-sensitive have failed to consistently reproduce the symptoms when MSG is administered with food. Reactions were sometimes triggered by a placebo, suggesting expectation or other factors can play a role. For those who genuinely experience an intolerance, avoiding foods with high concentrations of added MSG is the most effective way to prevent symptoms.
When Other Ingredients Are the True Culprit
When diarrhea occurs shortly after consuming a meal containing MSG, the monosodium glutamate itself is often mistakenly identified as the sole cause. Foods that contain added MSG are frequently characterized by other components known to trigger digestive distress. A significant factor is the high sodium content of these foods, which is separate from the sodium provided by the MSG itself.
Excessive sodium intake can cause osmotic diarrhea, where the high concentration of salt in the intestine pulls extra water into the bowel. This excess fluid volume prevents proper water absorption in the colon, leading to loose stools. Furthermore, many MSG-containing meals are also high in fat, which is the slowest macronutrient to digest.
Unabsorbed fat collects in the large intestine, where it can stimulate the colon to release extra fluids, resulting in diarrhea. The combination of a large, high-fat, high-sodium meal can easily overwhelm the digestive system of any individual. This leads to symptoms that are incorrectly attributed to the flavor enhancer. Other common additives, like certain spices or fermentation byproducts, may also be the true source of the gastrointestinal reaction.